Most of us think of the needed cleaning power of detergents and soaps, but we never question what components are necessary for a detergent to function properly. It is only once the properties and characteristics of surfactants is studied that their impact on consumer and industrial detergents becomes apparent.
Definition
Surfactants are often thought of as only tension lowering agents, but this is not always the case. Surfactants are a subset of amphiphiles. Amphiphiles are substances that have a polar-apolar duality giving them a double affinity. They contain a polar group usually consisting of heteroatoms such as oxygen, sulfur, phosphorous, or nitrogen, as well as a non-polar hydrocarbon chain (mainly alkyl or alkyl benzenes). These two regions of an amphiphile gives it the ability to partially hydrophilic and partially lipophilic (hydrophobic). This duality causes such molecules to orient themselves at the interface between polar and non-polar solvents if they have equal portions of hydrophilic and lipophilic components. Surfactants express this orienting characteristic and in addition cause the surface (or interface) tension to decrease. Amphiphiles (and surfactants) demonstrate other qualities which help distinguish them from one another.
Classification
One of the main uses of surfactants is in detergent formulas. Surfactants are classified into three major categories according to their behavior of dissociation in water; anionic surfactants, nonionic surfactants, and cationic surfactants.
Anionic surfactants dissociate in water completely to yield a negatively charged anion and a positively charged cation. The cation is generally a quaternary ammonium or alkaline metal such as potassium or sodium. This category of surfactants includes alkyl benzene sulfonates which are primarily used in detergents.
Nonionic surfactants, on the other hand, do not completely dissociate in water due to the incapability of their hydrophilic portion to dissociate from the lipophilic portion. Examples of these hydrophilic groups contained in the surfactant are alcohol, ether, ester, amide or phenol groups. Many of these nonionic surfactants hydrophilicity is due to the existence of a polyethylene glycol chain (from polycondensation of ethylene oxide. The lipophilic group is, like anionic surfactants, usually an alkyl or alkyl benzene.
Cationic surfactants do dissociate in water to form a positively charged cation and negatively charged anion, however these differ from the anionic surfactants because the ion that is amphiphilic is the cation, whereas in the anionic surfactants the anion is amphiphilic. Another distintion between these two types of surfactants is their anion. Cationic surfactants anion is almost always a halogen, whereas this is not the case for anionic surfactants (they have varying anion contributers). Along with compositional differences cationic surfactants are generally more costly than anionic ones because of their more laborious synthesis process that involves a high pressure hydrogenation reaction. This high cost causes their uses to be limited to only things that cannot use a substitute such as in bactericides.
There are also cases when a surfactant can exhibit both cationic and anionic tendencies. These molecules are called amphoteric (zwitter ions). Examples of amphoteric surfactant groups are the betaines and the sulfobetaines.
Figure 1. Dodecyl Betaine
Within amphoteric surfactants there are those that are not pH dependent, while others behave cationic in acidic media, anionic in alkali media, and amphoteric at intermediate pH levels.
History of Production
The production of surfactants has been increasing rapidly since the 1940’s. Originally the production of surfactants was mainly soaps using fatty acid salts. Then after World War II short olefins (C2-C3) were formed as by-products of catalytic cracking and used in the petroleum refining industry. In the early 50’s ethylene was used in styrene manufacture. Although previously propylene didn’t have a use, its low cost and polymerization possibilities made it a suitable substitute for alkyl groups made from fatty acids. This was a huge step for surfactants as this marked the birth of synthetic detergents that used alkyl benzene sufonate. These detergents were quickly excepted as a replacement for fatty acid soaps for domestic uses. In the 1960’s, however, the waste water from cities were often riddled with persistent foams which caused ecological damage by preventing adequate photosynthesis and oxygen dissolution. The cause was alkylbenzen sulfonate’s branched configuration which was not easily biodegraded. This caused the formation of laws prohibiting the use of propylene-based alkylates.
This law caused manufacturers to find different methods using different raw materials to produce linear alkylates. These new techniques included ethylene polymerization, molecular sieve extraction, and Edeleanu process through a urea-parafin complex, which were all more costly than the previous processes. Other advances included the development of steam cracking in the 60’s which yielded ethylene. Ethylene was used as a raw material for polymers and helped lower the cost of nonionic surfactants.
The main explosion in developments in surfactant synthesis techniques was during the 1970’s, which lead to surfactants use not only domestically but industrially as well. While nonionic surfactants since then have been used the most, several manufacturers now make and sell cationic and amphoteric surfactants. However, their use is limited to approximately five percent of the total market share of surfactants due to their greater comparative cost. Figure 2. Market shares of Surfactants (1990)
Of the world production of soaps, detergents and other surfactants that totaled 40 million tons in 2000, a quarter corresponds to North American production.
Raw Materials
Raw materials used to synthesize surfactants vary greatly, coming from different reactions with their prices ranging from one dollar per pound to more than twenty dollars per pound. The surfactant synthesis reactions can be as simple as hydrolysis to as complicated as a high pressure multiple part process.
The vast types of surfactant raw materials are classified by their origins. The large variety stems from the lipophilic group formation of the surfactant. The materials can be natural or synthesized.
Triglycerides are formed by the esterification. They are found in most vegetable oils and fats from animals. The opposite hydrolysis reaction causes separation of the polyalcohol (such as glycerol) from the fatty acids. The five most common fatty acids in triglycerides are palmitic acid (C10:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3). Oleic acid is the major component in animal fat and vegetable oil. Fatty acids with carbon chains ranging from twelve to eighteen carbons are very important in the manufacture of soaps due to their biocompatible lipophilic group. Figure 3. Opposite of Esterification
Other oils such as lignin and its derivatives are derived from wood (rosin and tall oils). These oils are not used in detergents or soaps, but rather used as dispersants for solid particles, in drilling fluids for example.
Several surfactant raw materials come from petroleum refining. These were used to lower the cost, especially for detergent surfactants. These components consist of a hydrocarbon lipophilic group containing between twelve and eighteen carbon atoms, and are from atmospheric distillation and catalytic cracking of gasoline and kerosene. These materials include alkylates for alkyl benzene production, linear paraffins, olefins, alkylates, and aromatics.
Alkylates for alkyl benzenes were produced by catalytic cracking. Short chain olefins such as propylene are branched compounds yielding a high octane number. This raw material was used to produce the surfactants lipophilic region by polymerization. The olefin is used in a Friedel-Crafts alkylation reaction that produces alkyl benzene which is sulfonated and neutralized forming a detergent type alkyl-benzene sulfonate. This process is much more cost effective than the process of making soap from natural oil or fat. The drawback of this process is the olefin alkylate is branched making it much less biodegradable than a linear compound. This degradation problem caused most countries to ban these “hard alkylates.” Figure 4. Trimerization of Propylene
Linear parrafins, olefins and alkylates can be formed by two processes. The first entails dividing them from petroleum containing a conglomeration of both mixed and isomerized substances. This process is done by synthesis through ethylene oligomerization.
Handmade Software, Inc. Image Alchemy v1.13
Figure 5. Process of ethylene oligomerization.
The second process involves polymerizing ethylene using the Ziegler oligomerization process. This mechanism involves forming a chain of ethylene polycondensation reactions on a template of triethyl-aluminum. Figure 6. Ziegler oligomerization to produce n-alkenes
Finally, aromatics are compounds incorporating benzene. The most common surfactant in powdered detergents is alkyl-benzene sulfonate. In the 70’s and 80’s most liquid dishwashing detergents used ethozylated alkyl-phenols, but in the recent past toxicity has caused the production of these surfactants to decrease and be replaced by other alcohol substitutes.
Surfactant Intermediate Chemicals
Ethylene oxide and ethoxylated alcohols are intermediate chemicals involved in the synthesis of surfactants. Of these two, ethoxylated alcohols are involved in detergents and hand dishwashing products.
Linear alcohols are used in the preparation of alkyl-ester-sulfates used in detergents. These alcohols contain carbon chains with a range of twelve to sixteen carbon atoms. These could be synthesized by controlling the hydrogenation process of natural fatty acids. Two other synthetic processes are available, however, that are less costly and therefore more heavily relied on. One such process is the alpha olefin alcohol process that comprises of oxidizing a tri-alkyl aluminum complex and hydrolyzing the ether product. The second viable process is the OXO process which is a hydroformylation of an olefin by reducing a fatty alcohol. This yields a combination of both primary and secondary alcohols. Figure 7. Primary (normal) and secondary (iso) alcohols produced by OXO process
Anionic Surfactants
There are many anionic surfactants that contribute to detergents. Among these are soaps (and other carboxylates), sulfates and sulfonates.
The term soap dictates a fatty acid sodium or potassium salt. The acid is a carboxylic acid while the metal ion may also be replaced by a metallic or organic cation compound. Soponification of triglycerides yields soaps. Figure 8: Saponification producing sodium stearate.
This reaction involves the hydrolysis of triglyceride at high temperature and pressure in the presence of a ZnO catalyst. Then the acids and glycerol separate into an oil phase and aqueous phase respectively. The acids are then distilled to distinguish their varying lengths before fractioning the C10-C12. This total process allows the maker to control the mixture of acids to make proper soap.
The acids of this process are chosen according to the intended use of the soap. Luxury soap bars used to be made strictly from vegetable oils, but not tallow (with a similar composition) has been used to make similar soaps by saponification, providing a cheaper cost of production. These soaps have between sixteen and eighteen carbons, which do not irritate skin but are not as water soluble and often leave white (calcium) deposits when used with hard water. Conversely, soaps with between fourteen and sixteen carbons have greater foaming capabilities and dissolve better even in hard water. These are usually added for these reasons. Transparent soaps are made from caster oil containing 12-hydroxy-oleic acid, while sweet soaps are distinctive due to the remaining glycerol in their formula.
An important part of this process is the hydroxide present which neutralizes the acid in presence of water. Also, this causes the pH of the soap solution to be very high in strongly alkaline solutions, helping increase its cleaning capabilities. This demonstrates that the cation (hydroxide) chosen will help determine the soaps solubility and cleaning power. It is note worthy that copper soap exhibits fungicidal properties.
Sulfates and sulfonates are also anionic surfactants used in detergents. Alkyl benzene sulfonic acid is synthesized by sulfonation of an aromatic ring following an electrophilic substitution mechanism. Sulfonic acid is strong and thus will completely dissociate in solution.
Alkyl ester monosulfuric acid is produced by sulfation, which is an esterification process of an alcohol by sulfuric acid (or anhydride). Sulfonates are then produced by neutralizing these products with hydroxide. This final alkyl-sulfate product still has traces of alcohol present due to the esterification hydrolysis reactions being in equilibrium. Alkyl-sulfates are most notable for their foaming and wetting capabilities and use in detergents.
Examples are lauryl sulfates (20 carbons) as a sodium, ammonium or ethanolamine salt. Sodium lauryl sulfate, for example, is extremely hydrophilic and therefore works well with water. The hydrophilicity can be lowered however with use of a longer carbon skeleton chain.
Another example of a sulfate surfactant is glyceride sulfates produced by the hydrolysis of glyceride with sulfuric acid present as shown below. This produces the sulfate and alcohol simultaneously. Figure 9. Hydrolysis of glyceride
Sulfonates have longer carbon chains (30-40 carbons). Current sulfonation reactions produce petroleum sulfonates. During this process it is important to limit the sulfonation to one sycle. This is done by decreasing sulfonic agent concentrations to make it the limiting reagent. This in turn causes the final product to consist of unsulfonated oil as well as sulfonated oil which is used in many industries including for detergents. Figure 10. LAS examples.
LAS are equally as effective as the branched chain alkyl benzene sulfonates but are not as effective foaming agents or emulsifiers. Maximum detergency with use of LAS are those with twelve or thirteen carbon atoms. Now detergent formulas contain more LAS than branched chain alkyl benzene sulfonates.
Dodecyl benzene sulfonates were later manufactured by a process including Friedel-Crafts alkylation followed by sulfonation and neutralization. An alkylate with approximately twelve carbons usually derived from a propylene tetramer makes us the commercial alkyl benzene sufonate used in synthetic detergents. These synthetic detergents began to replace soaps in domestic uses because of their enhanced hard water compatibility, cheaper price and better detergent capabilities. The disadvantage was lakes and rivers where waste water was deposited demonstrates a persistent foam layer due to its lack of biodegradability. This again, led to its baning causing manufacturers to switch to linear alkyl benzene sulfonates. These linear alkyl benzene sulfonates (LAS) were still cheap and could be used for most powdered detergents, with a range of ten to sixteen carbons with a benzene ring attached anywhere on the carbon chain (mainly attached to 3-6 carbon atom).
Hydrotropes are non-surfactant amphiphiles compounds which enhance solubility properties of another compound. Some of these are short chain alkyl benzene sulfonates used in liquid detergents. Hydrotropes are also used in powdered detergents to enhance the ability to attract and hold water molecules away from the surrounding environment, thus enhacing the detergency in dishwashing and fabric washing liquids.
Most LAS are alpha olefins which are then sulfonated to produce alpha olefin sulfonates. These have not replaced alkyl benzene sulfonates because they are not as effective detergents (although they do have increased hard water tolerance).
Additionally, sulfo-carboxylic compounds, which have two or more hydrophilic groups are found in slightly alkaline soaps. The most common example of these is sodium lauryl sulfoacetate.
Other anionic surfactants include sarcosides whose base is a cheap synthetic amino acid. Acid acylation with fatty acid chloride results in a surfactant with a lipophilic fatty amide group. These are used as bactericide and is compatible with anionic surfactants for dry shampoos for carpets and fabrics. Figure 11. Acid acylation resulting in sarcoside.
Nonionic Surfactants
Nonionic surfactants are used because they are highly compatible and are less sensitive to electrolytes enabling them to be used with hard water. Nonionic surfactants are god detergents and some even have foaming properties. Good solubility in water is ensures by the presence of at least 4 ethylene oxide groups.
Ethoxylated linear alcohols have structures dependent on the utilized alcohol. Primary alcohols (hydroxyl group at the end of carbon chain) are usually made by hydrogenation of fatty acids in catalytic hydrogenolysis, however Ziegler hydroformylation (OXO process) can also be utilized. Secondaty alcohols (hydroxyl group on second carbon) are produced by hydration of alpha olefins in aqueous sulfuric solvent. The most common alcohol used is tridecanol which has an ethoxylation degree between six and ten for detergents.
Ethoxylated alkyl-phenols are also used in the process of surfactant production. These can be produced by alkylating the phenol by Friedel-Crafts or adding an alpha-olefin to a benzene. Detergents utilized eight and nine carbon phenols with ethoxylation degrees between eight and twelve. Those with ethoxylation degrees greater than twenty only behave as detergents at high temperatures in high saline solutions.
Restrictions of intermediates due to toxicity issues lead to the process of eliminating the benzene ring completely. This was done by the replacement with ethoxylated linear alcohols. The only disadvantage to this solution is their decreased detergency capabilities when compared to phenol equivalents.
Thiols can also be ethoxylated forming very good detergent products that are utilized in industry applications only because of the need for proper disposal. These thiols also have high solubility in both aqueous and organic solvents making them even better industrial detergents.
Fatty acid esters are also nonionic surfactants used in detergents. The class of fatty acid esters that are utilized for detergents are acid ethoxylated fatty acids formed by the condensation of ethylene oxide producing polyethoxy esters. Polyethoxy esters of fatty acids are essentially the cheapest type of nonionic surfactant. They are not, however, very good detergents or foaming agents and cannot be used in high pH solutions. This are added to detergent formulas to decrease the total cost.
Nitrogenated nonionic surfactants, such as tertiary amine oxides are used as foaming agents. The polarization of the nitrogen-oxygen bond results in a negatively charged oxygen atom which can attract a proton when in aqueous medium. This causes amine oxides to form (cationic hydroxylamine). Most amine oxides contain one long chain and two short alkyls. Some hate two amine oxide groups with N-ethanol groups for increased foaming capabilities. These are used in hand dish-washing detergents.
Cationic Surfactants
The most notable among the cationic surfactants used for detergents are benzalkonium and alkyl trimethyl ammonium chloride (or bromide). These are highly utilized as antiseptic agents, disinfectants and sterilizing agents. They are used to fight corrosion in detergents.
Other (Less Common) Detergent Surfactants
Occationally fatty acid alkanol amides are used for foaming and wetting agents in dish detergent. An example of this is diethanol-lauryl amide.
Detergents ranging from floor cleaner to dishwashing are necessary in our world. With a annual sale total well over one billion thousand pounds of detergent, the surfactant market is continually growing. Without surfactants detergents would not be efficient in leaving floors (and dishes) clean and streakless. The progression of the processes of synthesizing detergents has evolved to resolve toxicity issues and decrease total cost.
References:
Barnes, J.R., J.P. Smit, J.R. Smit, P.G. Shpakoff, K.H. Raney, and M.C. Puerto, Development of Surfactants for Chemical Flooding at Difficult Reservoir Conditions, Paper SPE (Society of Petroleum Engineers)-113313-MS
Becher, P., “Emulsions: Theory and Practice”, Krieger Pub. (1977)
Cahn, A., Arno Cahn Consulting, Services Inc., Pearl River, NY “Surfactants and Detergents Calender” Journal of the American Oil Chemists’ Society. Volume 65, Issue 10, pA1696. DOI: 10.1007/BF02912605
Davidsohn A. et Mildwidsky B., “Synthetic Detergents”, Halsted Press. (1978)
Hayes, D. G.,Knovel (Firm). (2009). Biobased surfactants and detergents: Synthesis, properties, and applications. Urbana, IL: AOCS Press.
Jungerman E., Editor “Cationic Surfactants”, Marcel Dekker, New York (1970)
Lange, K. R. (1999). Surfactants: A practical handbook. Munich: Hanser.
Linfield W. M., Editor, “Anionic Surfactants”, Marcel Dekker, New York (1970)
McCutcheon, “Detergents and Emulsifiers”, McCutcheon Division Pub. Co.
O’Lenick A. J., “Surfactants and Detergents: Silicon based surfactants”. Paul D.I. Fletcher, Marie-Pierre Krafft, Reinhard Strey .Surfactants. Current Opinion in Colloid & Interface Science, Volume 10, Issues 3–4, October 2005, Pages 87 http://dx.doi.org.ezproxy2.library.drexel.edu/10.1016/j.cocis.2005.07.001
Schick M. J.. Editor “ Nonionic Surfactants”, Marcel Dekker, New York (1988)
Most of us think of the needed cleaning power of detergents and soaps, but we never question what components are necessary for a detergent to function properly. It is only once the properties and characteristics of surfactants is studied that their impact on consumer and industrial detergents becomes apparent.
Definition
Surfactants are often thought of as only tension lowering agents, but this is not always the case. Surfactants are a subset of amphiphiles. Amphiphiles are substances that have a polar-apolar duality giving them a double affinity. They contain a polar group usually consisting of heteroatoms such as oxygen, sulfur, phosphorous, or nitrogen, as well as a non-polar hydrocarbon chain (mainly alkyl or alkyl benzenes). These two regions of an amphiphile gives it the ability to partially hydrophilic and partially lipophilic (hydrophobic). This duality causes such molecules to orient themselves at the interface between polar and non-polar solvents if they have equal portions of hydrophilic and lipophilic components. Surfactants express this orienting characteristic and in addition cause the surface (or interface) tension to decrease. Amphiphiles (and surfactants) demonstrate other qualities which help distinguish them from one another.
Classification
One of the main uses of surfactants is in detergent formulas. Surfactants are classified into three major categories according to their behavior of dissociation in water; anionic surfactants, nonionic surfactants, and cationic surfactants.
Anionic surfactants dissociate in water completely to yield a negatively charged anion and a positively charged cation. The cation is generally a quaternary ammonium or alkaline metal such as potassium or sodium. This category of surfactants includes alkyl benzene sulfonates which are primarily used in detergents.
Nonionic surfactants, on the other hand, do not completely dissociate in water due to the incapability of their hydrophilic portion to dissociate from the lipophilic portion. Examples of these hydrophilic groups contained in the surfactant are alcohol, ether, ester, amide or phenol groups. Many of these nonionic surfactants hydrophilicity is due to the existence of a polyethylene glycol chain (from polycondensation of ethylene oxide. The lipophilic group is, like anionic surfactants, usually an alkyl or alkyl benzene.
Cationic surfactants do dissociate in water to form a positively charged cation and negatively charged anion, however these differ from the anionic surfactants because the ion that is amphiphilic is the cation, whereas in the anionic surfactants the anion is amphiphilic. Another distintion between these two types of surfactants is their anion. Cationic surfactants anion is almost always a halogen, whereas this is not the case for anionic surfactants (they have varying anion contributers). Along with compositional differences cationic surfactants are generally more costly than anionic ones because of their more laborious synthesis process that involves a high pressure hydrogenation reaction. This high cost causes their uses to be limited to only things that cannot use a substitute such as in bactericides.
There are also cases when a surfactant can exhibit both cationic and anionic tendencies. These molecules are called amphoteric (zwitter ions). Examples of amphoteric surfactant groups are the betaines and the sulfobetaines.
Figure 1. Dodecyl Betaine
Within amphoteric surfactants there are those that are not pH dependent, while others behave cationic in acidic media, anionic in alkali media, and amphoteric at intermediate pH levels.
History of Production
The production of surfactants has been increasing rapidly since the 1940’s. Originally the production of surfactants was mainly soaps using fatty acid salts. Then after World War II short olefins (C2-C3) were formed as by-products of catalytic cracking and used in the petroleum refining industry. In the early 50’s ethylene was used in styrene manufacture. Although previously propylene didn’t have a use, its low cost and polymerization possibilities made it a suitable substitute for alkyl groups made from fatty acids. This was a huge step for surfactants as this marked the birth of synthetic detergents that used alkyl benzene sufonate. These detergents were quickly excepted as a replacement for fatty acid soaps for domestic uses. In the 1960’s, however, the waste water from cities were often riddled with persistent foams which caused ecological damage by preventing adequate photosynthesis and oxygen dissolution. The cause was alkylbenzen sulfonate’s branched configuration which was not easily biodegraded. This caused the formation of laws prohibiting the use of propylene-based alkylates.
This law caused manufacturers to find different methods using different raw materials to produce linear alkylates. These new techniques included ethylene polymerization, molecular sieve extraction, and Edeleanu process through a urea-parafin complex, which were all more costly than the previous processes. Other advances included the development of steam cracking in the 60’s which yielded ethylene. Ethylene was used as a raw material for polymers and helped lower the cost of nonionic surfactants.
The main explosion in developments in surfactant synthesis techniques was during the 1970’s, which lead to surfactants use not only domestically but industrially as well. While nonionic surfactants since then have been used the most, several manufacturers now make and sell cationic and amphoteric surfactants. However, their use is limited to approximately five percent of the total market share of surfactants due to their greater comparative cost.
Figure 2. Market shares of Surfactants (1990)
Of the world production of soaps, detergents and other surfactants that totaled 40 million tons in 2000, a quarter corresponds to North American production.
Raw Materials
Raw materials used to synthesize surfactants vary greatly, coming from different reactions with their prices ranging from one dollar per pound to more than twenty dollars per pound. The surfactant synthesis reactions can be as simple as hydrolysis to as complicated as a high pressure multiple part process.
The vast types of surfactant raw materials are classified by their origins. The large variety stems from the lipophilic group formation of the surfactant. The materials can be natural or synthesized.
Triglycerides are formed by the esterification. They are found in most vegetable oils and fats from animals. The opposite hydrolysis reaction causes separation of the polyalcohol (such as glycerol) from the fatty acids. The five most common fatty acids in triglycerides are palmitic acid (C10:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3). Oleic acid is the major component in animal fat and vegetable oil. Fatty acids with carbon chains ranging from twelve to eighteen carbons are very important in the manufacture of soaps due to their biocompatible lipophilic group.
Figure 3. Opposite of Esterification
Other oils such as lignin and its derivatives are derived from wood (rosin and tall oils). These oils are not used in detergents or soaps, but rather used as dispersants for solid particles, in drilling fluids for example.
Several surfactant raw materials come from petroleum refining. These were used to lower the cost, especially for detergent surfactants. These components consist of a hydrocarbon lipophilic group containing between twelve and eighteen carbon atoms, and are from atmospheric distillation and catalytic cracking of gasoline and kerosene. These materials include alkylates for alkyl benzene production, linear paraffins, olefins, alkylates, and aromatics.
Alkylates for alkyl benzenes were produced by catalytic cracking. Short chain olefins such as propylene are branched compounds yielding a high octane number. This raw material was used to produce the surfactants lipophilic region by polymerization. The olefin is used in a Friedel-Crafts alkylation reaction that produces alkyl benzene which is sulfonated and neutralized forming a detergent type alkyl-benzene sulfonate. This process is much more cost effective than the process of making soap from natural oil or fat. The drawback of this process is the olefin alkylate is branched making it much less biodegradable than a linear compound. This degradation problem caused most countries to ban these “hard alkylates.”
Figure 4. Trimerization of Propylene
Linear parrafins, olefins and alkylates can be formed by two processes. The first entails dividing them from petroleum containing a conglomeration of both mixed and isomerized substances. This process is done by synthesis through ethylene oligomerization.
Figure 5. Process of ethylene oligomerization.
The second process involves polymerizing ethylene using the Ziegler oligomerization process. This mechanism involves forming a chain of ethylene polycondensation reactions on a template of triethyl-aluminum.
Figure 6. Ziegler oligomerization to produce n-alkenes
Finally, aromatics are compounds incorporating benzene. The most common surfactant in powdered detergents is alkyl-benzene sulfonate. In the 70’s and 80’s most liquid dishwashing detergents used ethozylated alkyl-phenols, but in the recent past toxicity has caused the production of these surfactants to decrease and be replaced by other alcohol substitutes.
Surfactant Intermediate Chemicals
Ethylene oxide and ethoxylated alcohols are intermediate chemicals involved in the synthesis of surfactants. Of these two, ethoxylated alcohols are involved in detergents and hand dishwashing products.
Linear alcohols are used in the preparation of alkyl-ester-sulfates used in detergents. These alcohols contain carbon chains with a range of twelve to sixteen carbon atoms. These could be synthesized by controlling the hydrogenation process of natural fatty acids. Two other synthetic processes are available, however, that are less costly and therefore more heavily relied on. One such process is the alpha olefin alcohol process that comprises of oxidizing a tri-alkyl aluminum complex and hydrolyzing the ether product. The second viable process is the OXO process which is a hydroformylation of an olefin by reducing a fatty alcohol. This yields a combination of both primary and secondary alcohols.
Figure 7. Primary (normal) and secondary (iso) alcohols produced by OXO process
Anionic Surfactants
There are many anionic surfactants that contribute to detergents. Among these are soaps (and other carboxylates), sulfates and sulfonates.
The term soap dictates a fatty acid sodium or potassium salt. The acid is a carboxylic acid while the metal ion may also be replaced by a metallic or organic cation compound. Soponification of triglycerides yields soaps.
Figure 8: Saponification producing sodium stearate.
This reaction involves the hydrolysis of triglyceride at high temperature and pressure in the presence of a ZnO catalyst. Then the acids and glycerol separate into an oil phase and aqueous phase respectively. The acids are then distilled to distinguish their varying lengths before fractioning the C10-C12. This total process allows the maker to control the mixture of acids to make proper soap.
The acids of this process are chosen according to the intended use of the soap. Luxury soap bars used to be made strictly from vegetable oils, but not tallow (with a similar composition) has been used to make similar soaps by saponification, providing a cheaper cost of production. These soaps have between sixteen and eighteen carbons, which do not irritate skin but are not as water soluble and often leave white (calcium) deposits when used with hard water. Conversely, soaps with between fourteen and sixteen carbons have greater foaming capabilities and dissolve better even in hard water. These are usually added for these reasons. Transparent soaps are made from caster oil containing 12-hydroxy-oleic acid, while sweet soaps are distinctive due to the remaining glycerol in their formula.
An important part of this process is the hydroxide present which neutralizes the acid in presence of water. Also, this causes the pH of the soap solution to be very high in strongly alkaline solutions, helping increase its cleaning capabilities. This demonstrates that the cation (hydroxide) chosen will help determine the soaps solubility and cleaning power. It is note worthy that copper soap exhibits fungicidal properties.
Sulfates and sulfonates are also anionic surfactants used in detergents. Alkyl benzene sulfonic acid is synthesized by sulfonation of an aromatic ring following an electrophilic substitution mechanism. Sulfonic acid is strong and thus will completely dissociate in solution.
Alkyl ester monosulfuric acid is produced by sulfation, which is an esterification process of an alcohol by sulfuric acid (or anhydride). Sulfonates are then produced by neutralizing these products with hydroxide. This final alkyl-sulfate product still has traces of alcohol present due to the esterification hydrolysis reactions being in equilibrium. Alkyl-sulfates are most notable for their foaming and wetting capabilities and use in detergents.
Examples are lauryl sulfates (20 carbons) as a sodium, ammonium or ethanolamine salt. Sodium lauryl sulfate, for example, is extremely hydrophilic and therefore works well with water. The hydrophilicity can be lowered however with use of a longer carbon skeleton chain.
Another example of a sulfate surfactant is glyceride sulfates produced by the hydrolysis of glyceride with sulfuric acid present as shown below. This produces the sulfate and alcohol simultaneously.
Figure 9. Hydrolysis of glyceride
Sulfonates have longer carbon chains (30-40 carbons). Current sulfonation reactions produce petroleum sulfonates. During this process it is important to limit the sulfonation to one sycle. This is done by decreasing sulfonic agent concentrations to make it the limiting reagent. This in turn causes the final product to consist of unsulfonated oil as well as sulfonated oil which is used in many industries including for detergents.
Figure 10. LAS examples.
LAS are equally as effective as the branched chain alkyl benzene sulfonates but are not as effective foaming agents or emulsifiers. Maximum detergency with use of LAS are those with twelve or thirteen carbon atoms. Now detergent formulas contain more LAS than branched chain alkyl benzene sulfonates.
Dodecyl benzene sulfonates were later manufactured by a process including Friedel-Crafts alkylation followed by sulfonation and neutralization. An alkylate with approximately twelve carbons usually derived from a propylene tetramer makes us the commercial alkyl benzene sufonate used in synthetic detergents. These synthetic detergents began to replace soaps in domestic uses because of their enhanced hard water compatibility, cheaper price and better detergent capabilities. The disadvantage was lakes and rivers where waste water was deposited demonstrates a persistent foam layer due to its lack of biodegradability. This again, led to its baning causing manufacturers to switch to linear alkyl benzene sulfonates. These linear alkyl benzene sulfonates (LAS) were still cheap and could be used for most powdered detergents, with a range of ten to sixteen carbons with a benzene ring attached anywhere on the carbon chain (mainly attached to 3-6 carbon atom).
Hydrotropes are non-surfactant amphiphiles compounds which enhance solubility properties of another compound. Some of these are short chain alkyl benzene sulfonates used in liquid detergents. Hydrotropes are also used in powdered detergents to enhance the ability to attract and hold water molecules away from the surrounding environment, thus enhacing the detergency in dishwashing and fabric washing liquids.
Most LAS are alpha olefins which are then sulfonated to produce alpha olefin sulfonates. These have not replaced alkyl benzene sulfonates because they are not as effective detergents (although they do have increased hard water tolerance).
Additionally, sulfo-carboxylic compounds, which have two or more hydrophilic groups are found in slightly alkaline soaps. The most common example of these is sodium lauryl sulfoacetate.
Other anionic surfactants include sarcosides whose base is a cheap synthetic amino acid. Acid acylation with fatty acid chloride results in a surfactant with a lipophilic fatty amide group. These are used as bactericide and is compatible with anionic surfactants for dry shampoos for carpets and fabrics.
Figure 11. Acid acylation resulting in sarcoside.
Nonionic Surfactants
Nonionic surfactants are used because they are highly compatible and are less sensitive to electrolytes enabling them to be used with hard water. Nonionic surfactants are god detergents and some even have foaming properties. Good solubility in water is ensures by the presence of at least 4 ethylene oxide groups.
Ethoxylated linear alcohols have structures dependent on the utilized alcohol. Primary alcohols (hydroxyl group at the end of carbon chain) are usually made by hydrogenation of fatty acids in catalytic hydrogenolysis, however Ziegler hydroformylation (OXO process) can also be utilized. Secondaty alcohols (hydroxyl group on second carbon) are produced by hydration of alpha olefins in aqueous sulfuric solvent. The most common alcohol used is tridecanol which has an ethoxylation degree between six and ten for detergents.
Ethoxylated alkyl-phenols are also used in the process of surfactant production. These can be produced by alkylating the phenol by Friedel-Crafts or adding an alpha-olefin to a benzene. Detergents utilized eight and nine carbon phenols with ethoxylation degrees between eight and twelve. Those with ethoxylation degrees greater than twenty only behave as detergents at high temperatures in high saline solutions.
Restrictions of intermediates due to toxicity issues lead to the process of eliminating the benzene ring completely. This was done by the replacement with ethoxylated linear alcohols. The only disadvantage to this solution is their decreased detergency capabilities when compared to phenol equivalents.
Thiols can also be ethoxylated forming very good detergent products that are utilized in industry applications only because of the need for proper disposal. These thiols also have high solubility in both aqueous and organic solvents making them even better industrial detergents.
Fatty acid esters are also nonionic surfactants used in detergents. The class of fatty acid esters that are utilized for detergents are acid ethoxylated fatty acids formed by the condensation of ethylene oxide producing polyethoxy esters. Polyethoxy esters of fatty acids are essentially the cheapest type of nonionic surfactant. They are not, however, very good detergents or foaming agents and cannot be used in high pH solutions. This are added to detergent formulas to decrease the total cost.
Nitrogenated nonionic surfactants, such as tertiary amine oxides are used as foaming agents. The polarization of the nitrogen-oxygen bond results in a negatively charged oxygen atom which can attract a proton when in aqueous medium. This causes amine oxides to form (cationic hydroxylamine). Most amine oxides contain one long chain and two short alkyls. Some hate two amine oxide groups with N-ethanol groups for increased foaming capabilities. These are used in hand dish-washing detergents.
Cationic Surfactants
The most notable among the cationic surfactants used for detergents are benzalkonium and alkyl trimethyl ammonium chloride (or bromide). These are highly utilized as antiseptic agents, disinfectants and sterilizing agents. They are used to fight corrosion in detergents.
Other (Less Common) Detergent Surfactants
Occationally fatty acid alkanol amides are used for foaming and wetting agents in dish detergent. An example of this is diethanol-lauryl amide.
Detergents ranging from floor cleaner to dishwashing are necessary in our world. With a annual sale total well over one billion thousand pounds of detergent, the surfactant market is continually growing. Without surfactants detergents would not be efficient in leaving floors (and dishes) clean and streakless. The progression of the processes of synthesizing detergents has evolved to resolve toxicity issues and decrease total cost.
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